Boilers have a structure for heating boiler water to generate vapor, and an anti-corrosive is applied to such boilers for preventing corrosion of metal members of the boilers. Particularly, in boilers for power generation, garbage incineration, etc., operated with a superheater or a steam turbine, ion-exchange water or desalinated water is generally used as a supplementary feed. Such boilers are generally operated at a concentration factor of about 30 to about 100, which is one of the water quality control factors. In such boilers, a phosphate salt, instead of a caustic alkali substance, is added so as to adjust the pH of boiler water for corrosion prevention, and a neutralizable amine or ammonia is added so as to elevate the pH of the boiler feed-condensate system, to thereby prevent elution of iron, whereby the amount of iron transferred into a boiler tank is reduced.
In recent years, however, the amounts of organic substances unintentionally transferred to a boiler tank have increased due to use of various water sources and impaired water quality. In addition, the blow rate has been reduced for the purpose of saving energy and water, and an organic oxygen scavenger is used instead of hydrazine. Under such circumstances, the pH of boiler water often lowers problematically. In order to solve the problem, the phosphate salt level of the boiler water is increased, or a phosphate salt-type boiler compounds (a sodium phosphate-sodium hydroxide mixture) having an Na/PO4 mole ratio of 3 or higher is used. In the above case, a phosphate salt hide-out phenomenon or alkali corrosion may problematically occur. The term “phosphate salt hide-out phenomenon” refers to deposition of the phosphate salt dissolved in boiler water caused by a rapid change in concentration or pressure. The hide-out phenomenon adversely affects a boiler member, and an example thereof is impairment of thermal conduction.
Some known anti-corrosives typically employed in the above boiler feed-condensate system include 2-aminoethanol (MEA) and monoisopropanolamine (MIPA). However, these compounds cannot fully elevate the pH of boiler water.
As an alternative to these compounds, Patent Document 1 discloses a boiler anti-corrosive containing methyldiethanolamine (MDEA). More specifically, Patent Document 1 discloses that the pH can be readily elevated by use of MDEA under high-temperature corrosive circumstances; that MDEA has low volatility to generate a less amount of vapor thereof, thereby less affecting the reaction system; and MDEA enhances the deoxygenation performance of an optionally employed oxygen scavenger, thereby realizing high anti-corrosive performance by addition thereof in a small amount.